How Much Water Should You Drink Based on Your Weight?
Official water guidelines are set by age and sex, not weight. Here's what the National Academies recommend, why “half your body weight in ounces” is a myth, and how to use your weight as a rough starting point.
It’s one of the most searched hydration questions there is: how much water should I drink for my weight? The honest answer is that no health body sets water needs by weight. The official guidelines are set by age and sex, and they already count the water you get from food.
Your size does affect how much you need, along with how active you are, the weather and your health. So weight can be a useful personal starting point. It just isn’t the rule the internet makes it out to be.
Want a number for you? Try our water intake calculator, which shows the official figure first and a personal estimate next to it.
What the Guidelines Actually Say
The National Academies of Sciences, Engineering, and Medicine set Adequate Intakes for water by age and sex. For adults 19 and older:
| Group | Total water a day (food + drinks) | From drinks |
|---|---|---|
| Women | 2.7 L (~91 fl oz) | ~2.2 L (~74 fl oz, ~9 cups) |
| Men | 3.7 L (~125 fl oz) | ~3.0 L (~101 fl oz, ~13 cups) |
| Pregnancy | 3.0 L (~101 fl oz) | ~2.3 L (~78 fl oz, ~10 cups) |
| Breastfeeding | 3.8 L (~128 fl oz) | ~3.1 L (~105 fl oz, ~13 cups) |
About 20% of the water people take in comes from food, which is why the drinks column is lower. Coffee, tea and other caffeinated drinks count toward the total too.
These are Adequate Intakes: the typical intakes of healthy people, not minimums you must hit. The National Academies also concluded that most healthy people meet their needs by letting thirst guide them, along with drinks at meals. For more of the numbers, see our women’s hydration statistics.
Why “Half Your Body Weight in Ounces” Is a Myth
No study or health body sets water needs as half your body weight in ounces. The rule ignores the roughly 20% of water that comes from food, and it ignores activity, climate and health. The National Academies set intakes by age and sex, not weight. Yet our study of Google’s hydration answers found the rule repeated as fact in page-one results.
Using Your Weight as a Rough Starting Point
Weight isn’t irrelevant. A larger body generally loses more water and needs more to replace it. That’s why the water intake calculator, and the goal in the Hydrate app, start from the guideline and add a little for a larger body.
Here’s how: the common calculator figure of about 35 ml per kilogram is an estimate of total water, food included. Take off the roughly 20% that food supplies and you get about 28 ml per kilogram to drink. Hydrate uses whichever is higher, that or the National Academies’ 74 fl oz, so nobody’s goal falls below the guideline:
| Weight | Water to drink a day (before activity) |
|---|---|
| 120 lbs | ~74 fl oz |
| 140 lbs | ~74 fl oz |
| 160 lbs | ~74 fl oz |
| 180 lbs | ~77 fl oz |
| 200 lbs | ~86 fl oz |
| 220 lbs | ~94 fl oz |
| 250 lbs | ~107 fl oz |
The weight part is an estimate, not a recommendation from any health body. Up to about 175 lbs it changes nothing: you get the guideline. Above that it adds a little. Use it as a starting point, then let thirst and how you feel fine-tune it.
What Moves Your Need Up
Whatever your starting point, these raise what you need:
Exercise: You lose water as sweat, so drink more on active days. How much depends on how long, how hard and how hot it is. Weighing yourself before and after a long workout is a practical way to see your own sweat losses.
Pregnancy: The Adequate Intake rises to about 101 fl oz of total water a day, about 78 fl oz of it from drinks. ACOG suggests 8 to 12 cups of water a day.
Breastfeeding: The Adequate Intake is about 128 fl oz of total water a day, about 105 fl oz from drinks, and 37 fl oz more than for other adult women.
Hot weather: The National Academies note that very active people in hot weather often need 6 liters (about 200 fl oz) or more of total water a day.
Illness: Fever, vomiting and diarrhea all increase fluid losses. Drink more, and check with a doctor if you can’t keep fluids down.
What About the 8-Glasses Rule?
The “8×8 rule” (eight 8 oz glasses a day, or 64 oz) is another single number for everyone. A 2002 review in the American Journal of Physiology found no scientific studies supporting it. It isn’t official advice, and it’s lower than the National Academies’ figures for drinks for both women and men. More on that in our hydration statistics.
So neither shortcut is the answer. Start from the guideline for your age and sex, adjust for your body and your day, and pay attention to thirst.
A Note on Over-Drinking
More water is not always better. Drinking a lot of plain water quickly, well beyond what your kidneys can clear, can cause hyponatremia, a dangerous drop in blood sodium. It’s rare in everyday life but has happened to endurance athletes who drank large amounts of plain water over several hours.
The goal is adequate hydration, not maximum hydration. Urine color is a handy everyday check: pale yellow usually means you’re well hydrated, and consistently clear may mean you’re drinking more than you need.
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